This invention relates generally to methods and devices for optical wavelength multiplexing and demultiplexing. More particularly, it relates to improved methods and devices for spatially separating and/or combining component wavelength beams in wavelength-division multiplexing optical communication.
In optical communications the technique of wavelength-division multiplexing (WDM) allows several different signals to be transmitted through a single optical fiber using a different wavelength for each signal. Implementation of WDM requires multiplexing techniques that spatially superimpose the different monochromatic beams prior to transmission, and demultiplexing techniques that spatially separate the different wavelength components of the polychromatic beam after transmission to recover the original monochromatic beams.
Demultiplexing requires a technique for dispersing or spatially separating light based on its wavelength. In optical communications applications, demultiplexing is often performed using conventional prisms or diffraction gratings to select and separate different wavelengths of light. These conventional devices, however, have small dispersion (change in propagation angle with respect to wavelength) and must consequently be quite large in order to achieve sufficient separation of the wavelengths. Smaller, integrated waveguide grating routers have been developed, but these integrated devices also suffer from relatively small effective angular deflection. As a result, these integrated devices must also be made relatively large. The gratings in these devices also make them complicated and expensive to fabricate. Demultiplexing is also performed using collections of cascaded thin-film filters, but these devices require many filters and are also complicated and expensive. Recently, Kosaka et al. (xe2x80x9cSuperprism phenomena in photonic crystals,xe2x80x9d Phys. Rev. B, Vol. 58, No. 16, Oct. 15, 1998; xe2x80x9cSelf-collimating phenomena in photonic crystals,xe2x80x9d Appl. Phys. Lett., Vol. 74, No. 9, Mar. 1, 1999; xe2x80x9cPhotonic crystals for micro-lightwave circuits using wavelength-dependent angular beam steering,xe2x80x9d Appl. Phys. Lett., Vol. 74, No. 10, Mar. 8, 1999) have proposed a method based on photonic crystals that can give angular dispersion many times larger than a prism or diffraction grating. Although the structures proposed by Kosaka et al. are much more compact than prior devices, their fabrication is expensive and complex.
Because the rapidly developing optical communications industry has a need for ever-smaller and inexpensive components, the above demultiplexing techniques and devices are inadequate for current and future demands. What is needed is a demultiplexing device that is both very compact and easily fabricated.
The present inventors have discovered a novel technique for separating light of differing wavelengths using a very simple and compact device. The inventors have discovered that certain simple integrated structures have high angular dispersion (i.e., exhibit the xe2x80x9csuperprism effectxe2x80x9d) at certain wavelengths and angles of incidence, and that these simple structures can therefore be used for optical demultiplexing. In one aspect of the present invention, a novel optical demultiplexing device comprises a compact integrated multilayered dielectric structure containing alternating layers of materials of different refractive indices. The multilayered stack of dielectric layers is comparable in structure to dielectric stacks used as mirrors, but is designed to operate just outside the main reflection region of the spectrum, rather than within the main reflection region as is normally the case. In this region just outside the main reflection region, there is strong group velocity dispersion, causing different wavelengths of light to travel at different angles through the dielectric stack. As a consequence, this property can be exploited to obtain a large beam-steering effect in the stack for wavelengths near the main reflection band edge.
The alternating layers of dielectric material can have periodic or non-periodic thicknesses. The device can be easily and inexpensively manufactured using well-known techniques, e.g., semiconductor epitaxial growth techniques, and techniques for fabricating dielectric thin film optical filters. The dielectric stack is preferably fabricated upon one or both sides of a transparent substrate so that the device can operate with light entering and/or exiting the substrate, eliminating the need for complicated antireflective coatings interfaced with the stack. Preferably, the device also comprises anti-reflective and reflective coatings to attain high optical efficiency and to increase spatial separation of the wavelengths of light, further reducing the size of the device. Because of the inherent scalability, the devices of the present invention can be designed to operate at any wavelength range of interest. Due to its inherent symmetry, in addition to operating as a demultiplexer, the device can also operate as a multiplexer. The present invention, therefore, exploits highly dispersive properties of multilayer dielectric stacks to provide a novel type of demultiplexer/multiplexer for WDM that is very compact, inexpensive to fabricate, and made from readily available materials.
In one aspect of the present invention, a solid-state optical wavelength division multiplexing-demultiplexing device is provided which comprises a multilayer dielectric stack which is typically fabricated on a substrate. The substrate may be partly or entirely detached from the stack, or the substrate may be left in place as part of the device. In some embodiments, the substrate material is transparent to light within a predetermined operating range of wavelengths, and is an optically functional part of the device. In other embodiments, the substrate simply provides mechanical support for the device. The multilayer dielectric stack has a substantially nonzero group velocity dispersion for a range of angles of incidence of light within a predetermined operating range of wavelengths just outside of a main reflection band edge of the stack. Consequently, the stack has substantially nonzero angular dispersions in the predetermined operating range of wavelengths. This property is preferably sufficient to ensure angular dispersions of the stack greater than 2 degrees per nm within 5 nm of the main reflection band edge. The multilayer dielectric stack may have a periodic or non-periodic structure. The stack may have a full photonic band gap, or it may have a photonic band gap for only a limited range of angles of incidence of light within the predetermined operating range of wavelengths. The multilayer dielectric stack may be composed of alternating layers of various common dielectric materials commonly used in the production of thin film dielectric stacks, as well as other dielectric materials. For example, the alternating layers may be composed of GaAs and AlGaAs grown on a GaAs substrate. Or the stack may be composed of alternating layers of GaAlAs and Al oxide grown on a GaAs substrate.
Some embodiments of the present invention may comprise one or more antireflective coatings and/or mirrors fabricated on the multilayer dielectric stack, on the substrate, or on both. Mirrors may be fabricated at the interface between the stack and substrate, as well as on the outside surfaces of the stack and/or substrate. In some embodiments of the device, two dielectric stacks are fabricated on opposing sides of the substrate, forming a device with a substrate sandwiched between two stacks.
In another aspect of the invention a method is provided for optical demultiplexing a polychromatic beam into spatially separated wavelength component beams. The method comprises coupling the polychromatic beam into a multilayer dielectric stack, separating the polychromatic beam into component beams having distinct component wavelengths as the polychromatic beam passes through the multilayer dielectric stack, and coupling the component beams out of the multilayer dielectric stack. The multilayer dielectric stack has a substantially nonzero group velocity dispersion within a predetermined operating range of wavelengths, and the distinct component wavelengths of the component beams are contained within the predetermined wavelength range. This property ensures that the wavelength components separate as they pass through the stack. The stack may have a periodic or non-periodic structure, and the operating range of wavelengths is preferably just outside a main reflection region, or photonic crystal band, of the stack. In some embodiments, the method also includes reflecting the component beams from a mirror in contact with the multilayer dielectric stack, and further separating the component beams as the component beams again pass through the multilayer dielectric stack. The method may also include transmitting the component beams and/or the polychromatic beam through antireflective material layers in contact with the multilayer dielectric stack. The method may also include transmitting the component beams and/or the polychromatic beam through an antireflective material layer in contact with a substrate material, and transmitting the component beams and/or the polychromatic beam through the substrate material, wherein the substrate material is in contact with the multilayer dielectric stack. The component beams and/or the polychromatic beam may also be reflected from a mirror in contact with the substrate material.